JP6444208B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile apparatus using an electrophotographic system or an electrostatic recording system.

  Conventionally, for example, in an electrophotographic image forming apparatus, an electrophotographic photosensitive member (photosensitive member) is uniformly charged and then exposed according to image information to form an electrostatic latent image on the photosensitive member. Is done. The electrostatic latent image is developed as a toner image using toner, and then transferred to a recording material such as a recording sheet by a transfer unit directly or via an intermediate transfer member. Thereafter, the toner image transferred to the recording material is heated and fixed on the recording material by a fixing unit. In general, the transfer unit forms a transfer nip with an image carrier such as a photosensitive member or an intermediate transfer member, and sandwiches and conveys a recording material in the transfer nip, and a transfer bias is applied to the toner image. Is transferred to a recording material. The fixing unit generally includes a heating member provided with a heating source and a pressure member pressed against the heating member. Then, the recording material is nipped and conveyed in a fixing nip formed at the contact portion between the heating member and the pressure member, and the toner image is fixed on the recording material by heating.

  There are various electrical resistances (volume resistance) of recording materials. For example, thin paper that absorbs moisture in a high temperature and high humidity environment, etc., or is dried in a low temperature and low humidity environment, etc. There is relatively high cardboard. Here, a recording material having a relatively high electrical resistance is also referred to as “high resistance paper”, and a recording material having a relatively low electrical resistance is also referred to as “low resistance paper”. However, the recording material is not limited to paper. In order to uniformly charge the recording materials having different electrical resistances, it is preferable to control the transfer bias at a constant current. However, when low-resistance paper such as paper that has absorbed moisture in a high-temperature and high-humidity environment is used, the transfer current escapes to the main body of the device such as the fixing means via the recording material, and good transfer is achieved with constant current control. It can be difficult. Therefore, the transfer bias is often controlled at a constant voltage in a high temperature and high humidity environment.

  On the other hand, as a fixing unit, there is a fixing unit in which a heating source has a resistance heating element as a heating element, and heat fixing is performed by heat generated by power supply to the resistance heating element. The power supply to the heating source is controlled according to the temperature detection result by the temperature detection element. As a method for supplying power to the heating source, there are a method using an AC voltage from a commercial power source and a method using the AC voltage rectified and converted into a DC voltage. In general, a method of supplying power from a commercial power supply does not require a rectifier circuit, and thus can be configured at a lower cost.

  When an AC voltage is applied to the heating source, the glass coating the heating element disposed on the substrate of the heating source acts as a capacitor on the equivalent circuit. Therefore, in this case, an AC voltage is applied to the fixing nip via the fixing film constituting the heating member.

  Here, since a recording material having a high water absorption rate has a low impedance, particularly when a low resistance paper such as a paper left in a high temperature and high humidity environment is used, an AC voltage applied to the fixing nip is passed through the recording material. Propagates to the transfer nip. In some cases, the transfer voltage fluctuates with the period of the alternating voltage oscillation, the transfer current becomes excessive or insufficient, and density unevenness (also referred to as “AC banding image”) occurs in the transfer image. Thus, in the configuration in which an AC voltage is applied to the recording material, an AC banding image may occur.

  In order to solve this problem, Patent Document 1 discloses a configuration in which the fixing unit is grounded via a capacitor. According to this configuration, it is possible to suppress the AC banding image by attenuating the AC voltage applied from the fixing nip to the transfer nip via the recording material.

JP 2006-195003 A

  However, in the above-described conventional configuration, when a surge occurs in the power supply voltage due to lightning or the like, most of the surge voltage is applied to the glass coating or temperature detection element of the heating source, and the glass coating or temperature detection element is destroyed. There is a case.

  To solve this problem, Patent Document 1 discloses a configuration in which a resistor is connected in series with a capacitor between the fixing unit and the ground potential to reduce the partial pressure of a surge voltage applied to a glass coating, a temperature detection element, or the like. Yes.

  However, when the resistors are connected in series in this way, the attenuation rate of the AC voltage due to the connection of the capacitor decreases, and the AC banding image cannot be sufficiently suppressed even if the capacitance of the capacitor is increased. There is.

On the other hand, the present inventors can connect a resistance element and a capacitive element in parallel between a conductive member that contacts the recording material sandwiched between the fixing nip and the transfer nip and the ground potential. And found to be effective in suppressing AC banding images. In this configuration, since the capacitor is not directly connected to the fixing unit, it is not necessary to connect a resistor in series with the capacitor in order to suppress the influence of the surge voltage due to lightning and the like as described above.

  However, as a result of further studies by the present inventors, in a job for continuously transferring a toner image to a plurality of recording materials, the transferability varies depending on the degree of charging of the capacitor connected to the conductive member, and the entire job It has been found that the uniformity of image quality may be reduced.

  Therefore, an object of the present invention is to suppress fluctuations in the transfer voltage due to application of an alternating voltage to the fixing unit, and to improve transferability through a plurality of recording materials when a toner image is continuously transferred to a plurality of recording materials. An object of the present invention is to provide an image forming apparatus capable of suppressing fluctuations.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image carrier that carries a toner image, a transfer unit that sandwiches a recording material between the image carrier and transfers the toner image from the image carrier to the recording material, and the transfer Applying means for applying a transfer bias for the transfer to the means, fixing means for holding the recording material and heating the recording material by applying an alternating voltage to fix the toner image on the recording material; A conductive member that contacts the recording material sandwiched simultaneously by a fixing sandwiching unit that sandwiches the recording material by the fixing unit, a transfer sandwiching unit that sandwiches the recording material by the image carrier and the transfer unit, and the conductive member A resistance element and a capacitance element connected in parallel between the first and second ground potentials, and a control means for controlling a transfer bias applied to the transfer means by the applying means, wherein the control means is one start Multiple records by instruction In the job for performing the transfer continuously, the transfer to at least one recording material from the (N + 1) th sheet onward with respect to the transfer bias for the transfer to the N (N is a natural number of 1 or more) recording material. The image forming apparatus is characterized in that the transfer bias is changed.

  According to the present invention, fluctuations in transfer voltage due to application of an AC voltage to the fixing unit are suppressed, and fluctuations in transferability through a plurality of recording materials when a toner image is continuously transferred to a plurality of recording materials. Can be suppressed.

1 is a schematic longitudinal sectional view of an image forming apparatus. 1 is a schematic diagram of a main part of an image forming apparatus according to Embodiment 1. FIG. 6 is a flowchart illustrating transfer bias control according to the first exemplary embodiment. 10 is a flowchart illustrating transfer bias control according to the second exemplary embodiment. 10 is a flowchart illustrating transfer bias control according to the third exemplary embodiment.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

[Example 1]
1. FIG. 1 is a schematic longitudinal sectional view of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 100 of this embodiment is an electrophotographic laser printer. FIG. 2 is a schematic diagram of a main part of the image forming apparatus of this embodiment.

  The image forming apparatus 100 includes a photosensitive drum 1 that is a drum-type (cylindrical) electrophotographic photosensitive member (photosensitive member) as an image carrier. The photosensitive drum 1 is configured by providing a photosensitive material such as OPC (organic optical semiconductor), amorphous selenium, or amorphous silicon on a cylindrical drum base made of aluminum, nickel, or the like. The photosensitive drum 1 is rotatably supported by the apparatus main body M, and is driven to rotate at a process speed (circumferential speed) of 230 mm / sec in the direction of arrow Rd in the figure by a driving source m1. In this embodiment, the outer diameter of the photosensitive drum 1 is 24 mm.

  Around the photosensitive drum 1, the following units are arranged in order along the rotation direction. First, a charging roller 2 which is a roller-shaped charging member as a charging unit is disposed. Next, an exposure device (laser scanner device) 3 as an exposure unit is arranged. Next, a developing device 4 as a developing unit is arranged. Next, a transfer roller 5 which is a roller-like transfer member as a transfer unit is disposed. The transfer roller 5 is an example of a transfer unit that sandwiches the recording material P with the photosensitive drum 1 and transfers a toner image from the photosensitive drum 1 to the recording material P when a voltage is applied thereto. Next, a cleaning device 6 as a cleaning means is arranged.

  In addition, a recording material cassette 7 containing a recording material (transfer material, recording medium) P such as paper is disposed in the lower part of the apparatus main body M in the drawing. Further, in the order from the recording material cassette 7 along the conveyance path of the recording material P, the feeding roller 8, the conveyance roller 9, the top sensor 10, the pre-transfer guide 17, the conveyance guide 11, the fixing device 12, the discharge sensor 13, and the conveyance roller. 14, a discharge roller 15 and a discharge tray 16 are disposed. Further, as will be described in detail later, the fixing device 12 is provided with a fixing inlet guide 18 and a fixing outlet roller 19.

  Next, an image forming operation will be described. The photosensitive drum 1 is rotationally driven in the direction of the arrow Rd in the figure by a driving source m1. The surface of the rotating photosensitive drum 1 is charged substantially uniformly by a charging roller 2 to a predetermined potential having a predetermined polarity (negative polarity in this embodiment). At this time, a charging bias (charging voltage) is applied to the charging roller 2 from a charging power source (high voltage power source) (not shown). The surface of the charged photosensitive drum 1 is subjected to image exposure L based on image information by an exposure device (laser optical system) 3, and the exposed portion of the charge is removed to form an electrostatic latent image (electrostatic image). It is formed. The electrostatic latent image formed on the photosensitive drum 1 is developed as a toner image by the developing device 4. The developing device 4 includes a developing roller 4 a as a developer carrying member that supplies toner to a portion (developing portion) facing the photosensitive drum 1. A developing bias (developing voltage) is applied to the developing roller 4a from a developing power source (high voltage power source) (not shown), whereby toner is attached to the electrostatic latent image on the photosensitive drum 1 and developed as a toner image. (Visualized). In this embodiment, the toner is a reversal developing method in which toner charged to the same polarity as the charging polarity of the photosensitive drum 1 is attached to the exposed portion where the absolute value of the potential is reduced by being exposed after being uniformly charged. An image is formed.

  The toner image formed on the photosensitive drum 1 is transferred to a recording material P such as paper by the action of the transfer roller 5. The transfer roller 5 is urged (pressed) toward the photosensitive drum 1 by a transfer pressure spring (not shown) that is an urging member serving as an urging means, and is pressed against the photosensitive drum 1. As a result, a transfer nip (transfer nipping portion) Nt is formed at the contact portion between the photosensitive drum 1 and the transfer roller 5. The transfer roller 5 rotates following the rotation of the photosensitive drum 1. In this embodiment, the outer diameter of the transfer roller 5 is 12.5 mm. The transfer roller 5 sandwiches and conveys the recording material P with the photosensitive drum 1. At this time, a transfer bias (transfer voltage) that is a DC voltage having a polarity opposite to the charging polarity (normal charging polarity) of the toner at the time of development is applied to the transfer roller 5 from the transfer power source (high voltage power source) 21. As a result, the toner image on the photosensitive drum 1 is transferred to a predetermined position on the recording material P.

  The recording material P is stored in the recording material cassette 7, fed one by one by the feed roller 8, transported by the transport roller 9, and transported to the transfer nip Nt along the pre-transfer guide 17 that is a guide member. Is done. At this time, the leading edge of the recording material P is detected by the top sensor 10 and synchronized with the toner image on the photosensitive drum 1.

  The recording material P on which the toner image is transferred is conveyed to the fixing device 12 along the conveyance guide 11 where the unfixed toner image is heated and pressurized and fixed on the surface of the recording material P. Details of the fixing device 12 will be described later.

  The recording material P on which the toner image has been fixed by the fixing device 12 is conveyed by a conveying roller 14 and discharged by a discharge roller 15 onto a discharge tray 16 formed on the upper surface of the apparatus main body M in the drawing. At this time, the trailing edge of the recording material P is detected by the discharge sensor 13 in order to confirm whether or not there is a jam (paper jam).

  On the other hand, the photosensitive drum 1 after the toner image is transferred to the recording material P is removed by the cleaning blade 6a of the cleaning device 6 after the toner (transfer residual toner) that is not transferred to the recording material P and remains on the surface thereof is removed. It is used for image formation.

  By repeating the above operation, image formation can be performed one after another.

2. Next, the fixing device 12 will be described. In this embodiment, the fixing device 12 is a pressure roller driving type fixing device having a fixing film made of a flexible endless belt.

  More specifically, the fixing device 12 includes a fixing film 12a which is a film-like fixing rotating body. The fixing device 12 has a pressure roller 12b as a pressure member in contact with the fixing film 12a. Further, the fixing device 12 includes a ceramic heater (hereinafter, also simply referred to as “heater”) 12 c as a heating source for heating the toner via the fixing film 12 a. The fixing device 12 includes a heater holder 12d that is a support member that supports the heater 12c. The fixing member 12a, the heater 12c, the heater holder 12d, and the like constitute a heating member.

In this embodiment, the fixing film 12a has a substantially cylindrical shape when not deformed, and its outer diameter is 18 mm. More specifically, the fixing film 12a has the following three-layer structure. First, it has a cylindrical base film with a thickness of 30 to 80 μm in which a heat conductive filler is dispersed in a polyimide resin. Further, a primer layer having a volume resistivity of 1 × 10 ⁵ Ω · cm or less and a thickness of 1 to 6 μm, in which conductive carbon is dispersed in a fluororesin, is formed on the surface. Furthermore, a release layer having a thickness of 5 to 20 μm in which a conductivity imparting substance is dispersed in a fluororesin is formed thereon.

  In this embodiment, the heater 12c is configured by printing a resistance heating element made of a silver alloy as a heating element on an alumina substrate and applying a glass coating on the surface of the resistance heating element. The heater 12c is provided with a thermistor (not shown) as a temperature detection element. The heater 12c generates heat when an AC voltage is applied to a resistance heating element as a heating element from an AC power source (AC voltage source, heater driving power source) 20 which is a commercial power source in this embodiment. Supply of electric power to the heater 12c and temperature adjustment of the heater 12c are performed by a control unit 150 that is provided in the apparatus main body M and comprehensively controls the operation of the image forming apparatus 100.

  In this embodiment, the outer diameter of the pressure roller 12b is 20 mm. In the pressure roller 12b, a heat-resistant elastic layer 12f having elasticity such as silicone rubber is provided on the outer peripheral surface of a metal core 12e, and a material having high releasability such as fluororesin is provided on the outermost layer. The used release layer (not shown) is provided. The pressure roller 12b is pressed by a pressure spring (not shown) which is an urging member, and presses the fixing film 12a against the heater 12c from below in the figure by the outer peripheral surface of the release layer. As a result, a fixing nip (fixing pinching portion) Nf is formed at the contact portion between the fixing film 12a and the pressure roller 12b.

  When the pressure roller 12b is rotationally driven by the drive source m1 in the direction of the arrow R12b in the drawing, the rotational force is applied to the fixing film 12a due to the pressure friction between the pressure roller 12b and the fixing film 12a in the fixing nip Nf. Works. The fixing film 12a rotates in the direction of the arrow R12a in the figure following the rotation of the pressure roller 12b, while the inner peripheral surface of the fixing film 12a slides in close contact with the surface of the heater 12c facing downward in the figure. .

  In addition, electric power is supplied to the heater 12c, the heater 12c is heated to rise to a predetermined temperature, and the temperature-controlled state is maintained. In this state, the recording material P carrying an unfixed toner image is introduced between the fixing film 12a and the pressure roller 12b in the fixing nip Nf. Thus, in the fixing nip Nf, the surface of the recording material P carrying the toner image is in close contact with the outer peripheral surface of the fixing film 12a, and the recording material P is moved together with the fixing film 12a between the pressure roller 12b and the heater 12c. It is sandwiched between and transported. In this nipping and conveying process, the heat of the heater 12c is applied to the recording material P via the fixing film 12a, and the unfixed toner image on the recording material P is heated and pressurized on the recording material P to be melted and fixed. . The recording material P that has passed through the fixing nip Nf is separated in curvature from the fixing film 12a.

  Here, as described above, the glass (glass coating) on which the resistance heating element is coated in the heater 12c is electrically regarded as a capacitor, and its capacity is about several hundred pF (200 pF to 400 pF). Accordingly, the AC voltage from the AC power source 20 is transmitted from the resistance heating element to the fixing nip Nf through the gas coating. In this embodiment, the recording material P may be sandwiched simultaneously between the fixing nip Nf and the transfer nip Nt in the conveyance path of the recording material P.

  In this embodiment, a fixing inlet guide 18 as a guide member for guiding the recording material P is provided on the downstream side of the transfer nip Nt and the upstream side of the fixing nip Nf in the conveyance direction of the recording material P. The fixing inlet guide 18 serves as a guide when the recording material P is conveyed from the transfer nip Nt to the fixing nip Nf. The fixing inlet guide 18 contacts the recording material P and regulates its conveyance path, and guides the recording material P to the fixing nip Nf. The fixing inlet guide 18 is an example of a conductive member that comes into contact with the recording material P that is simultaneously held between the fixing nip Nf and the transfer nip Nt. In other words, the fixing entrance guide 18 is an example of a conductive member (conductive member) other than the photosensitive drum 1 that contacts the recording material P when the recording material P is in contact with the photosensitive drum 1. In this embodiment, the fixing inlet guide 18 is made of metal as a conductive material.

  In the present embodiment, a fixing outlet roller (pair) 19 that sandwiches and conveys the recording material P is provided on the downstream side of the fixing nip Nf in the conveying direction of the recording material P. In this embodiment, the fixing outlet roller 19 is configured by forming an elastic layer on the outer periphery of the core metal, the elastic layer is made of conductive rubber, and the core metal is electrically grounded.

  In this embodiment, a fixing bias (DC bias) is applied to the fixing film 12a from the bias power source 30 via the limiting resistor 31. When a negative DC voltage is applied from the bias power source 30 to the fixing film 12a, an electric field is generated in the fixing nip Nf in a direction that urges the negative toner toward the recording material P. As a result, the offset or scattering of the toner image on the fixing film 12a at the fixing nip Nf can be suppressed.

  In this embodiment, the fixing device 12 includes the fixing film 12a, the pressure roller 12b, the heater 12c, the heater holder 12d, the fixing inlet guide 18, the fixing outlet roller 19, and the like. The fixing device 12 is an example of a fixing unit that sandwiches the recording material P and applies the AC voltage to heat the recording material P to fix the toner image on the recording material P.

3. Control Mode In this embodiment, a control unit 150 serving as a control unit provided in the image forming apparatus 100 performs overall control of the image forming apparatus 100. The control unit 150 includes a CPU that is a central element that performs arithmetic processing, and a memory such as a ROM and RAM that are storage elements. The RAM stores sensor detection results, calculation results, and the like, and the ROM stores control programs, data tables obtained in advance, and the like. Each control object in the image forming apparatus 100 is connected to the control unit 150. In particular, in relation to the present embodiment, the transfer power source 21 and a current detection circuit 40 as current detection means are connected to the control unit 150. The current detection circuit 40 can detect a DC current value flowing through the transfer roller 5 when the transfer power source 21 applies a DC voltage to the transfer roller 5. In this embodiment, the transfer power source 21 is configured to output a constant voltage having a voltage value set by the control of the control unit 150. Then, the control unit 150 changes the set value of the output of the transfer power supply 21 so that the current value detected by the current detection circuit 40 becomes a predetermined current value, thereby causing the transfer roller 5 to transfer the predetermined value. A voltage for supplying the current can be applied. Further, the control unit 150 can acquire information on the voltage value and the current value from the set value of the output of the transfer power supply 21 and the detection result of the current detection circuit 40 at this time, respectively.

  Here, the image forming apparatus 100 performs a “job” that is a series of printing operations (image output operations) that is started by one start instruction and forms and outputs an image on a single or a plurality of recording materials P. . In general, a job includes an image forming process (printing process), a pre-rotation process, a paper gap process when images are formed on a plurality of recording materials P, and a post-rotation process. The image forming process is a period for forming an electrostatic latent image, forming a toner image, and transferring a toner image of an image that is actually formed and output on the recording material P. Say. The pre-rotation process is a period for performing a preparatory operation before the image forming process from when the start instruction is input until the actual image formation is started. The inter-sheet process is a period corresponding to the interval between the recording material P and the recording material P when performing “continuous printing” in which images are continuously formed on a plurality of recording materials P. The post-rotation process is a period during which an organizing operation (preparation operation) after the image forming process is performed. The non-image forming period is a period other than the image forming time, and is a preparatory operation at the time of turning on the power of the image forming apparatus 100 or returning from the sleep state. This is included during the previous multi-rotation process.

  In the present embodiment, the image forming apparatus 100 can change the sheet interval (print interval) during continuous printing. For example, the sheet interval (print interval) is changed by a plurality of image forming modes provided in accordance with the type of recording material P used for image formation. Here, the distance between the recording material P and the recording material P on the transport path during continuous printing is referred to as “inter-paper”. In addition, the time required for the sheet to pass through a predetermined position (for example, the contact start position between the fixing entrance guide and the recording material) during continuous printing is referred to as “print interval”. More specifically, in the present embodiment, during continuous printing, as an example, the recording material P is discharged at a printing speed of 47 sheets per minute with a sheet interval (print interval) of 12 mm (0.05 seconds). In this embodiment, as another example, during continuous printing, the recording material P can be discharged at a printing speed of 23 mm (0.1 second) between sheets (print interval).

4). 2. Suppression of AC Banding Image As shown in FIG. 2, in this embodiment, a capacitor 23 as a capacitive element and a resistor 22 as a resistive element are connected in parallel between the fixing inlet guide 18 and the ground potential. . That is, the fixing inlet guide 18 is connected to one terminal of each of the capacitor 23 and the resistor 22, and the other terminal of each of the capacitor 23 and the resistor 22 is connected to the ground potential (ground). The capacitor 23 increases the AC component of the current It flowing from the fixing inlet guide 18 to the ground potential and suppresses the AC banding image. The resistor 22 is for keeping the DC component of the current It flowing from the fixing inlet guide 18 to the ground potential within a certain range to suppress the transfer missing image and the charging of the fixing inlet guide 18. Thereby, charging of the fixing entrance guide 18 can be suppressed, and the potential generated by rubbing against the recording material P can be lowered.

  Here, in order to keep the surface potential of the fixing film 12a below a certain level, the cored bar 12e of the pressure roller 12b may be electrically grounded via a resistor. In this case, when the recording material P enters the fixing nip Nf, the transfer current escapes to the fixing nip Nf, and the transfer property at the transfer nip Nt differs before and after the recording material P enters the fixing nip Nf, so that the entire image is uniform. It may not be possible to obtain a good transferability. For this reason, it is not desirable to make the resistance value of the resistor 22 too large in order to achieve both the suppression of the missing transfer image and the uniformity of the image. That is, in order to suppress a transfer omission image due to transfer current escape via the fixing inlet guide 18, it is desirable that the resistance value of the resistor 22 be increased to a certain level or more. However, in order to ensure the uniformity of the image, the resistance value may be reduced below a certain value so as to reduce the difference in the amount of transfer current escaped before and after the recording material P enters the fixing nip Nf. desired. As described above, the resistor 22 connected between the fixing inlet guide 18 and the ground potential has an optimum resistance value according to the specific configuration of the image forming apparatus 100 to which the present embodiment is applied.

  In the image forming apparatus 100 of this embodiment, the resistance value of the high voltage resistance is changed to 20 MΩ, 30 MΩ, 40 MΩ, and 50 MΩ as the resistor 22 connected between the fixing inlet guide 18 and the ground potential, and the transfer omission image and the scattered image are scattered. The situation of outbreaks was examined. As a result, when the resistance value is 30 MΩ or less, although the occurrence of the scattered image due to the charging of the fixing inlet guide 18 can be suppressed, the effect of suppressing the transfer missing image due to the escape of the transfer current is insufficient. On the other hand, at a resistance value of 50 MΩ, a density difference due to a difference in transferability before and after entering the fixing nip Nf occurred. Therefore, it was found that the optimum resistance value of the resistor 22 is 40 MΩ in the image forming apparatus 100 of the present embodiment.

  Further, the larger the capacitance of the capacitor 23 connected between the fixing entrance guide 18 and the ground potential, the more effective the suppression of the AC banding image. Since this capacitor 23 is only indirectly connected to the heater 12c via the recording material P, in order to suppress the surge voltage caused by lightning strikes from being excessively applied to the glass coating or the like of the heater 12c, There is no need to connect a resistor in series. And even if it does not provide such a resistor connected in series, the capacity | capacitance of the capacitor | condenser 22 can be made as high as possible. In the image forming apparatus 100 of this embodiment, the capacitance of the capacitor 23 was changed to 4700 pF, 10000 pF, 30000 pF, and 47000 pF, and the state of occurrence of an AC banding image was confirmed. As a result, the higher the capacitance of the capacitor 23, the higher the effect on the AC banding image.

  From the above, by connecting the 40 MΩ resistor 22 and the 47000 pF capacitor 23 in parallel between the fixing inlet guide 18 and the ground potential, an AC banding image and a good image without transfer omission and scattering are obtained. It was confirmed that it was obtained. In the present embodiment, the example in which the resistor 22 and the capacitor 23 are connected in parallel to the fixing inlet guide 18 has been described. However, the same effect can be obtained when the resistor 22 and the capacitor 23 are connected to a pre-transfer guide provided on the upstream side of the transfer nip Nt. can get.

5). As described above, the AC banding image can be suppressed by connecting the resistor 22 and the capacitor 23 in parallel between the fixing inlet guide 18 and the ground potential. However, in this configuration, in a job in which toner images are continuously transferred onto a plurality of recording materials P, transferability varies depending on the degree of charging of the capacitor 23, and uniformity of image quality in the entire job is reduced. I found out that

  That is, for example, when using low resistance paper such as paper that has absorbed moisture in a high-temperature and high-humidity environment, the transfer current escapes until charge is accumulated in the capacitor 23 via the recording material P. At this time, for example, if the width of the transfer bias that is actually output and the minimum transfer bias necessary for transferring the toner image onto the recording material P are not sufficiently secured, the transfer current escapes. The current required for transfer may be insufficient. In addition, when the electric charge is accumulated in the capacitor 23 more than a certain amount, escape of the transfer current to the capacitor 23 is reduced. Therefore, the transferability may be different before and after the electric charge is accumulated in the capacitor 23. As a result, it may be difficult to obtain uniform image quality throughout a series of printing operations (jobs) from the start of printing to the end of printing.

In order to improve the uniformity of image quality in the entire job, it is effective to change the transfer bias before and after the electric charge is accumulated in the capacitor 23. Table 1 shows good transferability before and after accumulation of electric charge in the capacitor 23 (initial stage of job: first sheet in this embodiment) and after accumulation (after the initial stage of job: second sheet and later in this embodiment). The result of examining the obtained transfer current (required transfer current value) is shown. The transferability was confirmed by performing continuous printing in a normal temperature environment (23 ° C., 50%) using Canon A4 size paper OceRedLabel (basis weight 80 g / m ² ). The interval between sheets (print interval) was 12 mm (0.05 seconds). The “open paper” is paper immediately after the wrapping paper is opened, and is an example of high resistance paper such as paper having a moisture content of 5% or less. The “left paper” is paper that has been left in a high temperature and high humidity environment for 48 hours, for example, and is an example of low resistance paper such as paper having a moisture content of 7% or more.

  From Table 1, the first sheet of the job is the open paper, the transfer current is insufficient when the transfer current is 4 μA or less, and transfer failure occurs. When the transfer current is 8 μA or more, the transfer current is too large, It can be seen that retransfer occurs where the charge is reversed and transferred again onto the photosensitive drum 1. Therefore, for open paper, good transferability can be obtained at a transfer current of 5 to 7 μA. On the other hand, since the transfer current escapes to the capacitor 23 via the fixing inlet guide 18 in the case of the leaving paper, the necessary transfer current is larger than that of the open paper, and good transferability can be obtained when the transfer current is 7 to 9 μA. I understand.

  As for the second and subsequent sheets, it can be seen that good transferability is obtained when the transfer current is 4 to 6 μA on open paper, and good transferability is obtained when the transfer current is 5 to 7 μA on the untreated paper. . In this way, the necessary transfer current value for the second and subsequent sheets is reduced from that of the first sheet because the transfer current transiently flows through the capacitor 23 when the first sheet is passed, so that charges are accumulated and the subsequent transfer is performed. This is because the current escape is reduced, and this tendency is particularly remarkable in the case of leaving paper.

  From the results in Table 1, when the print interval (paper interval) is 0.05 seconds (12 mm), the first sheet of the job can be opened by performing constant current control with a transfer current of 7 μA, and the second and subsequent sheets with a transfer current of 6 μA. It can be seen that good transferability can be obtained for both the direct paper and the standing paper.

  Here, the electric charge stored in the capacitor 23 in a section where the recording material P is in contact with the fixing entrance guide 18 is discharged in a period corresponding to the interval between sheets. However, as the gap between the sheets becomes narrower, the electric charge stored in the capacitor 23 becomes less likely to be discharged between the sheets.

Table 2 shows the result of examining the necessary transfer current value by performing continuous printing while changing the printing interval (paper interval). The transferability was confirmed by continuous printing using A4 size paper OceRed Label (basis weight 80 g / m ² ) manufactured by Canon under normal temperature environment (23 ° C., 50%).

  From Table 2, it can be seen that if the print interval (interval) is increased to 0.1 second (23 mm), the required transfer current value is the same for the first and second and subsequent sheets of the job. This is because the charge accumulated in the capacitor 23 in the first print is released as the print interval (between papers) is widened, and the transfer current between the first and second sheets escapes to be the same. This is because. That is, in this embodiment, it is understood that it is not necessary to change the transfer current between the first sheet and the second sheet if the print interval (between sheets) is 0.1 second (23 mm) or more.

  As described above, when the print interval (paper interval) is less than the predetermined value, the transfer bias is changed between the initial stage of the job (for example, the first sheet) and the subsequent period (for example, the second sheet or later). Good transferability can be obtained throughout the job, and the uniformity of image quality can be improved. If the print interval (paper interval) is variable as in this embodiment, it is possible to control whether or not the transfer bias needs to be changed according to the print interval (paper interval).

  The degree to which the transfer current escapes (that is, the degree to which the capacitor 23 is charged) varies depending on the electrical resistance of the recording material P. That is, when high resistance paper is used, the transfer current is difficult to escape and the capacitor 23 is difficult to be charged. On the other hand, when low resistance paper is used, the transfer current easily escapes and the capacitor 23 is easily charged. Therefore, in this embodiment, when the print interval (paper interval) is less than a predetermined value, the transfer bias is changed at the initial stage of the job (for example, the first sheet) and thereafter (for example, the second sheet or later). Whether it is necessary or not is controlled in accordance with information on the electrical resistance of the recording material P.

6). Transfer Bias Control Next, with reference to the flowchart of FIG. 3, an example of the transfer bias control of this embodiment, that is, control for changing the target value (target transfer current) of the transfer current between the first and second sheets of the job. explain.

  First, upon receiving a print signal (job start signal), the control unit 150 sets the target transfer current to 7 μA during the pre-rotation process (Step 101), and the first sheet of the job performs constant current control with the target transfer current of 7 μA. Perform (Step 102). Next, the control unit 150 determines whether or not the designated number of prints has been reached (Step 103), and ends the job if it has been reached. Further, if the designated number of prints has not been reached, the control unit 150 determines whether the page interval between the previous page and the next page is equal to or greater than a predetermined value (0.1 second in this embodiment) as a threshold ( It is determined whether the capacitor 23 is charged) (Step 104). When the page interval is equal to or greater than the predetermined value, the control unit 150 proceeds to Step 101 and performs constant current control with a target transfer current of 7 μA because the capacitor 23 is discharged.

  On the other hand, when the page interval is less than the predetermined value in Step 104, the control unit 150 performs the following. That is, since the capacitor 23 is charged, it is further determined whether the transfer voltage of the immediately previous page is equal to or lower than a predetermined value (800 V) as a threshold (determining whether the recording material P of the immediately previous page is high resistance paper or low resistance paper). (Step 105). In this embodiment, it is known that when the transfer voltage is 800 V or less when the constant current control is performed with the target transfer current of 7 μA, it can be determined that the sheet is low resistance paper, and when it exceeds 800 V, it is determined that the sheet is high resistance paper. When the transfer voltage when the constant current control is performed with the target transfer current of 7 μA exceeds the predetermined value, the control unit 150 performs the following. That is, since the recording material P of the immediately preceding page is a high-resistance paper having a high resistance value such as the open paper or the second surface of the double-sided print, and the capacitor 23 is discharged, the process proceeds to Step 101, and a constant current is reached at a target transfer current of 7 μA. Take control. In addition, when the transfer voltage of the immediately previous page is equal to or lower than a predetermined value, the control unit 150 performs the following. That is, the previous page is a low-resistance paper having a low resistance value such as a neglected paper, and the capacitor 23 is charged. Therefore, the target transfer current is lowered to 6 μA (Step 106), and constant current control is performed (Step 102). . Thereafter, the same operation is repeated until the designated number is reached.

  By such transfer bias control, appropriate transfer bias control according to the state of the electric charge of the capacitor 23 can be performed.

  In this embodiment, the case where the transfer bias is controlled at a constant current has been described. However, the same effect can be obtained by changing the transfer voltage by controlling the transfer bias at a constant voltage. In this case, the target transfer voltage is reduced in correspondence with the reduction of the target transfer current in this embodiment. In this embodiment, the example in which the transfer current is changed between the first sheet and the second and subsequent sheets has been described. However, the transfer current (or transfer voltage) can be changed stepwise. Typically, the transfer current (or transfer voltage) for the later recording material P in the job can be changed stepwise so as to decrease. For example, the charging state of the capacitor 23 is changed so that the second transfer bias is changed with respect to the first transfer bias of the job, and the third transfer bias is changed with respect to the second transfer bias. The transfer bias can be gradually changed according to the above. Further, it is possible not to further change the transfer bias for the recording material P after the capacitor 23 has been charged more than a certain amount. Also in this case, as in the present embodiment, when the gap between the recording material P and the immediately preceding recording material P becomes long, or when the electrical resistance of the immediately preceding recording material P is higher than a certain value (when the capacitor 23 is discharged). In addition, for example, the transfer bias can be appropriately returned to the first sheet according to the discharge state. In this embodiment, the transfer bias for the second and subsequent sheets of the job is changed with respect to the transfer bias for the first sheet of the job as the initial stage of the job. However, the present invention is not limited to this. For example, in a configuration in which the capacitor 23 is not sufficiently charged up to an arbitrary number of sheets at the beginning of the job, the recording material P after the next recording material P with respect to the transfer bias for the recording material P of the arbitrary number of sheets. Alternatively, the transfer bias may be changed.

  As described above, in the present exemplary embodiment, the image forming apparatus 100 transfers the resistance element 22 and the capacitance element 23 connected in parallel between the conductive member 18 and the ground potential, and the transfer applied to the transfer unit 5 by the applying unit 21. Control means 150 for controlling the bias. Then, the control unit 150 performs the following control in a job (continuous print job) in which transfer is continuously performed on a plurality of recording materials according to one start instruction. That is, the transfer bias for transfer to at least one recording material from the (N + 1) th sheet is changed with respect to the transfer bias for transfer to the Nth (N is a natural number of 1 or more) recording material P. In particular, in the present embodiment, the control means 150 controls the transfer bias at a constant current, and at least one sheet of recording after the (N + 1) th sheet is greater than the current value of the transfer bias for the transfer to the Nth recording material P. The transfer bias current value for transfer to the material P is reduced. Alternatively, the control unit 150 controls the transfer bias at a constant voltage so that the transfer bias voltage value for at least one recording material P after the (N + 1) th sheet is higher than the voltage value of the transfer bias for transfer to the Nth sheet recording material P. Therefore, the voltage value of the transfer bias may be reduced.

  In the present embodiment, the control unit 150 determines whether the transfer bias needs to be changed based on the interval between each of the (N + 1) th and subsequent recording materials P and the preceding recording material P in a continuous print job. Control no. That is, when the interval is less than a predetermined value, the transfer bias for transfer to each of the N + 1th and subsequent recording materials P is changed with respect to the transfer bias for transfer to the Nth recording material P. To do. On the other hand, when the interval is equal to or greater than the predetermined value, the transfer bias for transfer to each of the N + 1th and subsequent recording materials P is set to the transfer bias for transfer to the Nth recording material P. Do not change. Further, in this embodiment, the control unit 150 changes the transfer bias based on a value correlated with the electrical resistance of the recording material P preceding the respective recording materials P for the (N + 1) th and subsequent sheets in a continuous print job. The necessity of the control is controlled. In this embodiment, the value correlated with the electric resistance of the recording material P is a voltage value when the transfer bias is controlled at a constant current. That is, when the value correlated with the electrical resistance exceeds a predetermined value in the direction in which the electrical resistance is low, the transfer bias for the transfer to the respective recording materials P after the (N + 1) th sheet is set as the Nth recording material. Change to transfer bias for transfer to P. On the other hand, if the value correlated with the electric resistance does not exceed the predetermined value, the transfer bias for the transfer to the recording material P after the (N + 1) th sheet is set as the transfer bias for the Nth recording material P. Therefore, the transfer bias is not changed. Here, in this embodiment, N is 1, but the present invention is not limited to this. However, N is usually 5 or less, typically 3 or less.

  As described above, in this embodiment, the resistor 22 and the capacitor 23 are connected in parallel to the conductive member that contacts the recording material P sandwiched between the transfer nip Nt and the fixing nip Nf such as the fixing inlet guide 18. As a result, it is possible to obtain an AC banding image and a good image free from transfer omission and scattering. At the same time, according to the present embodiment, by changing the transfer bias, it is possible to suppress image defects such as transfer defects and retransfers caused by excessive or insufficient transfer bias due to differences in the charge state of the capacitor 23.

[Example 2]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of the present embodiment are substantially the same as those of the first embodiment. Therefore, elements having the same or corresponding functions or configurations as those of the image forming apparatus according to the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  In this embodiment, the change amount of the transfer bias is changed according to the use environment of the image forming apparatus 100. In other words, in this embodiment, the difference in resistance value between the open paper and the left sheet is greatly different as in a high temperature environment, and the resistance value difference between the open paper and the left paper is small as in a low temperature environment, and the transfer current escapes. The transfer bias change amount is changed when the amount is small. Thereby, it is possible to set an appropriate transfer bias according to the use environment, and it is possible to obtain a good image free from transfer failure and retransfer.

  Differences in transferability depending on the use environment will be described with reference to Tables 3 and 4. Table 3 shows the result of examining the required transfer current value in a high temperature environment (30 ° C., 50%), and Table 4 shows the required transfer current value in a low temperature environment (15 ° C., 50%). The paper interval (print interval) was 12 mm (0.05 seconds).

  First, it can be seen from Table 3 that the required transfer current value differs greatly between open paper and untreated paper in a high temperature environment. This is because, since the resistance value of the left sheet is low, the escape of the transfer current to the fixing inlet guide 18 is large, and an apparent transfer current is required more than the open paper. Further, it can be seen that the required transfer current value for the second and subsequent sheets is almost the same as the required transfer current value for the first sheet in the open paper. On the other hand, in the case of a left sheet, electric charge is accumulated in the capacitor 23 due to the escape of the first transfer current, and the escape current after the second sheet is greatly reduced. Therefore, the necessary transfer current value for the second and subsequent sheets is 1. It can be seen that the required transfer current value for the first sheet is significantly different. Therefore, in a high temperature environment, good transferability can be obtained by performing constant current control with a transfer current of 9 μA for the first sheet and a transfer current of 7 μA for the second sheet and thereafter.

  Next, as shown in Table 4, open paper is used for transfer defects that occur due to insufficient transfer current in low temperature environments and punch-through images that occur due to excessive transfer current (image defects due to abnormal discharge at the transfer nip). It can be seen that there is almost no difference in the required transfer current value between the left and the left paper. This is because there is little difference in resistance value between the open paper and the leaving paper, and the resistance value is relatively high. That is, there is almost no escape of the transfer current to the fixing inlet guide 18 for both the open paper and the left paper, and there is no difference in the required transfer current value between the first sheet and the second and subsequent sheets. Therefore, in a low-temperature environment, good transferability can be obtained by performing constant current control with a transfer current of 6 μA regardless of whether the first sheet of the job is the second sheet or later.

  In this embodiment, the target transfer current is set as shown in Table 5 based on the results of the high temperature environment, the low temperature environment, and the room temperature environment of the first embodiment. In this embodiment as well, as in the first embodiment, whether or not the transfer bias needs to be changed is controlled according to information regarding the print interval (inter-paper) and the electrical resistance of the recording material P.

  Next, an example of transfer bias control in this embodiment will be described with reference to the flowchart of FIG. In the present embodiment, the image forming apparatus 100 is provided with an environmental temperature sensor 50 (FIG. 1) that detects the environmental temperature of the image forming apparatus 100 as an environment detection unit.

  First, when receiving a print signal (job start signal), the control unit 150 determines the usage environment of the image forming apparatus 100 based on the detection result of the environmental temperature sensor 50 during the pre-rotation process (Step 201). When the controller 150 determines that the environment is a high temperature environment, the controller 150 sets the target transfer current to 9 μA (Step 202). In addition, when the controller 150 determines that the room temperature environment is set, the controller 150 sets the target transfer current to 7 μA (Step 203). If the controller 150 determines that the environment is a low temperature environment, the controller 150 sets the target transfer current to 6 μA (Step 204). Then, the control unit 150 performs constant current control on the first sheet of the job with the set target transfer current (Step 205). Next, the control unit 150 determines whether or not the designated number of prints has been reached (Step 206). If the number has reached, the job is terminated, and if not, the use environment is determined again (Step 207). If the controller 150 determines that the temperature environment is high, the controller 150 determines whether the page interval between the previous page and the next page is equal to or greater than a predetermined value (0.1 seconds in this embodiment) (charge is accumulated in the capacitor 23). (Step 208). When the page interval is equal to or larger than the predetermined value, the control unit 150 proceeds to Step 201 and performs constant current control with a target transfer current (target transfer current 9 μA in a high temperature environment) according to the use environment. . On the other hand, when the page interval is less than the predetermined value, the control unit 150 determines whether the transfer voltage of the immediately previous page is equal to or lower than the predetermined value (600 V) as the threshold value because the capacitor 23 is charged (the previous page). Whether the recording material P is a high resistance paper or a low resistance paper) (Step 209). In this embodiment, in a high temperature environment (30 ° C., 50%), a low resistance paper is used if the transfer voltage is 600 V or less when constant current control is performed at a target transfer current of 9 μA, and a high resistance paper is used if the transfer voltage exceeds 600 V. It is known that it can be determined. When the transfer voltage when the constant current control is performed with the target transfer current of 9 μA exceeds the predetermined value, the control unit 150 performs the following. That is, since the recording material P of the immediately preceding page is high resistance paper (open paper / double-sided second surface, etc.) and the capacitor 23 is discharged, the process proceeds to Step 201 and the target transfer current (high temperature environment In this case, constant current control is performed with a target transfer current of 9 μA. In addition, when the transfer voltage of the immediately previous page is equal to or lower than a predetermined value, the control unit 150 performs the following. In other words, since the previous page is low resistance paper (such as left paper) and the capacitor 23 is charged, the target transfer current is lowered to 7 μA (Step 210), and the process proceeds to Step 205 and constant current control is performed with the set target transfer current. I do. Thereafter, the same operation is repeated until the designated number is reached.

  If the controller 150 determines that the room temperature environment is set at Step 207, the controller 150 determines whether the page interval between the previous page and the next page is equal to or greater than a predetermined value (0.1 seconds in this embodiment) (the capacitor 23 has a charge). (Step 211). When the page interval is equal to or larger than the predetermined value, the control unit 150 proceeds to Step 201 and performs constant current control with a target transfer current according to the usage environment (a target transfer current of 7 μA in a normal temperature environment). . On the other hand, when the page interval is less than the predetermined value, the control unit 150 determines whether the transfer voltage of the immediately previous page is equal to or lower than the predetermined value (800 V) as the threshold value because the capacitor 23 is charged (the previous page). Whether or not the recording material P is high resistance paper or low resistance paper) (Step 212). In this embodiment, in a normal temperature environment (23 ° C., 50%), a low resistance paper is used if the transfer voltage is 800 V or less when constant current control is performed at a target transfer current of 7 μA, and a high resistance paper is used if the transfer voltage exceeds 800 V. It is known that it can be determined. When the transfer voltage when the constant current control is performed with the target transfer current of 7 μA exceeds the predetermined value, the control unit 150 performs the following. That is, since the recording material P of the immediately preceding page is high resistance paper (open paper / double-sided second surface, etc.) and the capacitor 23 is discharged, the process proceeds to Step 201 and the target transfer current according to the use environment (in a normal temperature environment) Constant current control is performed with a target transfer current of 7 μA). In addition, when the transfer voltage of the immediately previous page is equal to or lower than a predetermined value, the control unit 150 performs the following. That is, since the previous page is low-resistance paper (such as left paper) and the capacitor 23 is charged, the target transfer current is lowered to 6 μA (Step 213), and the process proceeds to Step 205 and constant current control is performed with the set target transfer current. I do. Thereafter, the same operation is repeated until the designated number is reached.

  Moreover, the control part 150 is as follows, when it is judged that it is a low temperature environment by Step7. That is, since the recording material P has little moisture and there is almost no escape of the transfer current to the capacitor 23, the process proceeds to Step 205, and printing is continued until the specified number of sheets is reached without changing the target transfer current from 6 μA for the second and subsequent sheets. To do.

  By such transfer bias control, it is possible to perform appropriate transfer bias control according to the charge state of the capacitor 23 depending on the use environment.

  In this embodiment, the case where the transfer bias is controlled at a constant current has been described. However, as described in Embodiment 1, the same effect can be obtained even when the transfer bias is controlled at a constant voltage. In this embodiment, the example in which the transfer current is changed between the first sheet and the second and subsequent sheets has been described. However, as described in the first embodiment, the transfer current (or transfer voltage) is changed stepwise. It is possible. In this embodiment, the transfer bias for the second and subsequent sheets of the job is changed with respect to the transfer bias for the first sheet of the job at the beginning of the job. However, as described in the first embodiment, the present invention is limited to this. It is not a thing. In this embodiment, the example in which the change amount of the transfer current is changed according to the environmental temperature condition has been described. However, it can be combined with the humidity condition. That is, the change amount of the transfer current (transfer voltage) can be changed according to at least one of the environmental temperature and humidity.

  As described above, in this embodiment, the control unit 150 changes the transfer bias change amount based on the use environment of the image forming apparatus 100. In particular, in the present exemplary embodiment, the control unit 150 causes the transfer bias when the temperature of the environment in which the image forming apparatus 100 is used is the second temperature lower than the first temperature than when the temperature is the first temperature. Reduce the amount of change. Alternatively, the change amount of the transfer bias is smaller when the control unit 150 uses the second humidity lower than the first humidity than when the humidity of the environment in which the image forming apparatus 100 is used is the first humidity. You may make it do.

  As described above, according to the present embodiment, in addition to the effects of the first embodiment, by controlling the change amount of the transfer bias according to the use environment, the transfer failure caused by the excess or shortage of the transfer bias due to the difference in the charge state of the capacitor 23. And image defects such as retransfer can be suppressed.

[Example 3]
Next, still another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of the present embodiment are substantially the same as those of the first embodiment. Therefore, elements having the same or corresponding functions or configurations as those of the image forming apparatus according to the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  In this embodiment, when the constant current control is performed as the transfer bias control, the lower limit value (lower limit voltage value) of the transfer voltage is set so that the transfer voltage does not become lower than a predetermined value. In this transfer bias control, the lower limit voltage value is changed at the initial stage of the job (first sheet in the present embodiment) and the second half (second sheet and later in the present embodiment). That is, when low resistance paper such as left paper in a high temperature and high humidity environment is used, the transfer current escapes more and the transfer current may be insufficient. Therefore, when the transfer bias is controlled at a constant current, a lower limit voltage value is set so that the transfer voltage is not lower than the lower limit voltage value, so that a necessary transfer current can be secured. In this embodiment, when a recording material P having a large amount of transfer current escape, such as left paper in a high-temperature and high-humidity environment, is used, the transfer bias is automatically controlled at a constant voltage at the lower limit voltage value. Therefore, the control of the present embodiment does not require an environmental temperature sensor or the like as in the second embodiment, and can be realized at a relatively low cost.

  Next, the necessity of changing the lower limit voltage value between the first and second sheets of the job will be described. Table 6 shows the transferability with open paper in a high-temperature and high-humidity environment (30 ° C., 50%), and Table 7 shows the transfer property with standing paper in a high-temperature and high-humidity environment (30 ° C., 50%). The paper interval (print interval) was 12 mm (0.05 seconds).

  From Table 6, since the amperage of the transfer current to the fixing inlet guide 18 is small in the open paper, there is no large difference between the required transfer current values for the first and second sheets of the job, and the transfer current is 7 μA and constant current. It can be seen that good transferability can be obtained by controlling. In this embodiment, the transfer voltage value when the constant current control is performed with the transfer current of 7 μA on the open paper is about 1 kV.

  Next, it can be seen from Table 7 that the charge is not accumulated in the capacitor 23 in the case of the left paper, and the transfer current escapes to the fixing inlet guide 18, and the transfer voltage is 800 to 900 V in the first job, and good transferability is obtained. It turns out that it is obtained. Further, since the electric charge is accumulated in the capacitor 23 for the second and subsequent sheets of the job, there is little escape of the transfer current to the fixing inlet guide 18, so that good transferability is achieved with a transfer voltage of 600 to 700 V lower than the first sheet. It can be seen that

  That is, as the transfer bias control, constant current control is performed at a transfer current of 7 μA, and at that time, as the lower limit voltage value, the lower limit voltage value is 1: 800 to 900 V for the first job, and the lower limit voltage value for the second and subsequent sheets. 2: A lower limit voltage value of 600 to 700V is set. As a result, good transferability can be obtained regardless of the state of the recording material P (open / closed) and whether the first or second sheet of the job.

  Next, transfer bias control of this embodiment will be described with reference to the flowchart of FIG.

  First, when receiving a print signal (job start signal), the control unit 150 sets the first lower limit voltage value 1 (850 V) during the pre-rotation process (Step 301), and starts constant current control at a target transfer current of 7 μA. (Step 302). Next, the control unit 150 determines whether or not the voltage during the constant current control is equal to or higher than the lower limit voltage value 1 (Step 303). If the lower limit voltage value is 1 or more, the controller 150 uses high resistance paper (open paper / second side, etc.), and therefore performs constant current control with a target transfer current of 7 μA (Step 304). Further, when the control unit 150 is smaller than the lower limit voltage value 1, it is low resistance paper (eg, untreated paper), and therefore performs constant voltage control at the lower limit voltage value 1 (Step 305). Next, the control unit 150 determines whether the designated number of prints has been reached (Step 306). If the designated number of prints has been reached, the control unit 150 ends the job, and if not, determines whether the previous page was under constant current control (determines whether the capacitor 23 is charged) (Step 307). ). If the previous page is constant current control, the control unit 150 uses high resistance paper (open paper / second side, etc.), and the capacitor 32 is discharged. A value 1 is set to perform constant current control with a target transfer current of 7 μA. Further, the control unit 150 performs the following operation if the immediately preceding page is not constant current control (that is, constant voltage control). In other words, since the low resistance paper (left paper, etc.) is used, the capacitor 23 is charged, so the page interval between the next page and the next page is a predetermined value (0.1 in this embodiment). Seconds) or more (determining whether the capacitor 23 is charged) (Step 308). When the page interval is equal to or greater than the predetermined value, the control unit 150 proceeds to Step 301, sets the lower limit voltage value 1 and performs constant current control with the target transfer current 7 μA because the capacitor 23 is discharged. On the other hand, when the page interval is less than the predetermined value, the control unit 150 sets the lower limit voltage value 2 (650 V) because the capacitor 23 is charged (Step 309), and then proceeds to Step 302 to set the target transfer current at 7 μA. Perform constant current control. If the transfer voltage is less than the lower limit voltage value 2 at Step 303, the constant voltage control is performed at the lower limit voltage value 2. Thereafter, the same operation is repeated until the designated number is reached.

  By such transfer bias control, appropriate transfer bias control can be performed according to the charge state of the capacitor 23.

  As described above, in this embodiment, the control unit 150 performs constant current control of the transfer bias with a predetermined current value, and when the constant current control is performed with the predetermined current value, the voltage value becomes less than a predetermined lower limit voltage value. In this case, the transfer bias is controlled at a constant voltage with the lower limit voltage value. Further, the control unit 150 is configured to perform transfer to at least one recording material after the (N + 1) th sheet with respect to a lower limit voltage value of a transfer bias for transfer to the Nth recording material P in a continuous print job. Change the lower limit voltage value of the transfer bias. In particular, in the present embodiment, the control unit 150 in the continuous printing job, at least one recording material after the (N + 1) th sheet is lower than the lower limit voltage value of the transfer bias for the transfer to the Nth recording material P. The lower limit voltage value of the transfer bias for transfer with respect to P is reduced.

  As described above, according to the present embodiment, as in the first embodiment, an AC banding image and a good image free from transfer omission and scattering can be obtained. At the same time, according to the present embodiment, by changing the transfer bias according to the moisture absorption state of the recording material P, an image such as transfer failure or retransfer caused by transfer bias excess or deficiency due to the difference in the charge state of the capacitor 23. Defects can be suppressed.

[Others]
As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example.

  For example, in the above-described embodiments, the AC banding image has been described as being generated due to the AC voltage applied to the heater of the fixing device. However, if the AC voltage is applied to the recording material by having a heating source using an AC voltage as a drive source, the present invention can be similarly applied and the same effect can be obtained.

  In the above-described embodiments, the pre-transfer guide is used as the conductive member that contacts the recording material that is simultaneously held between the fixing holding portion and the transfer holding portion. However, the conductive member can take any form as long as it is in contact with the recording material sandwiched between the fixing sandwiching portion and the transfer sandwiching portion, and the same effect can be obtained. That is, by using a conductive member having a function of guiding the recording material or a function of conveying the recording material, a simple and inexpensive configuration can be easily realized without providing a separate conductive member. However, in order to obtain the effect of the present invention, it is not essential that the conductive member has a function of guiding the recording material or a function of conveying the recording material. In addition to or in place of the pre-transfer guide in the above-described embodiment, a guide member that guides or conveys the recording material or a conductive member that does not have the function of the conveyance member may be provided.

  In the above-described embodiment, the case where the influence of the AC voltage transmitted from the fixing nip to the transfer nip formed by the photosensitive member as the image carrier and the transfer member is suppressed has been described. However, the present invention is not limited to this. In order to suppress the influence of the AC voltage transmitted from the fixing nip to the transfer nip formed by the intermediate transfer member serving as the image carrier and the transfer member. Are equally applicable. That is, after the toner image formed on the photosensitive member as the first image carrier is transferred (primary transfer) to the intermediate transfer member as the second image carrier such as an endless belt, the intermediate image is transferred. There is an intermediate transfer type image forming apparatus that transfers (secondary transfer) a toner image from a transfer body to a recording material. Secondary transfer is performed by applying a secondary transfer voltage to a secondary transfer member that forms a transfer nip (secondary transfer nip) in contact with the intermediate transfer member. In this case, there is a concern that the AC voltage applied to the fixing device affects the secondary transfer voltage in the secondary transfer nip, causing image defects such as density unevenness. Therefore, regarding the arrangement of the conductive member and the connection of the capacitor and the resistor, the transfer nip in the above-described embodiment is replaced with the secondary transfer nip, and substantially the same configuration as that in the above-described embodiment is applied. Image defects can be suppressed.

  In addition, the present invention solves a common problem in monochrome / color image forming apparatuses, and the effect of this embodiment is not limited to monochrome image forming apparatuses, but is similar to color image forming apparatuses. It is obtained.

  Further, the transfer means is not limited to the roller-shaped transfer member, and may be any transfer member such as a brush shape or a blade shape.

1 Photosensitive drum 5 Transfer roller 12 Fixing device 12a Fixing film 12b Pressure roller 12c Ceramic heater 17 Pre-transfer guide 18 Fixing inlet guide 19 Fixing outlet roller 20 AC power source 21 Transfer power source 22 Resistor 23 Capacitor

Claims (15)

An image carrier for carrying a toner image;
Transfer means for sandwiching a recording material between the image carrier and transferring a toner image from the image carrier to the recording material;
Applying means for applying a transfer bias for the transfer to the transfer means;
Fixing means for fixing the toner image on the recording material by sandwiching the recording material and heating the recording material by applying an AC voltage;
A conductive member that is in contact with the recording material sandwiched at the same time by a fixing sandwiching unit that sandwiches the recording material by the fixing unit, and a transfer clamping unit that sandwiches the recording material by the image carrier and the transfer unit;
A resistive element and a capacitive element connected in parallel between the conductive member and a ground potential;
Control means for controlling a transfer bias applied to the transfer means by the application means;
Have
In the job for performing the transfer continuously on a plurality of recording materials in accordance with one start instruction, the control unit is configured to perform the transfer bias for the transfer on the Nth recording material (N is a natural number of 1 or more). An image forming apparatus, wherein a transfer bias for the transfer with respect to at least one recording material after the (N + 1) th sheet is changed.   The control means controls the transfer bias at a constant current, and the transfer bias for at least one recording material after the (N + 1) th recording material is greater than the current value of the transfer bias for the transfer with respect to the Nth recording material. 2. The image forming apparatus according to claim 1, wherein a current value of the transfer bias for reducing the current is small.   The control means controls the transfer bias at a constant voltage, and the transfer bias for at least one recording material after the (N + 1) th recording material is higher than the voltage value of the transfer bias for the transfer with respect to the Nth recording material. The image forming apparatus according to claim 1, wherein a voltage value of a transfer bias for reducing the voltage is small.   The image forming apparatus according to claim 1, wherein the control unit changes a transfer bias change amount based on a use environment of the image forming apparatus.   The control unit reduces the change amount of the transfer bias when the temperature of the environment in which the image forming apparatus is used is the first temperature, and when the second temperature is lower than the first temperature. The image forming apparatus according to claim 4.   The control unit reduces the change amount of the transfer bias when the humidity of the environment in which the image forming apparatus is used is the second humidity lower than the first humidity than when the humidity is the first humidity. The image forming apparatus according to claim 4. The control means performs constant current control of the transfer bias with a predetermined current value, and if the voltage value becomes less than a predetermined lower limit voltage value when the constant current control is performed with the predetermined current value, the transfer bias is set to the lower limit voltage value. With constant voltage control, and
In the job, the control unit is configured to perform the transfer with respect to at least one recording material after the (N + 1) th recording material with respect to the lower limit voltage value of the transfer bias for the transfer with respect to the Nth recording material. The image forming apparatus according to claim 1, wherein the lower limit voltage value of the transfer bias is changed.   In the job, the control unit is configured to perform the transfer for at least one recording material after the N + 1th recording material than the lower limit voltage value of the transfer bias for the transfer with respect to the Nth recording material. The image forming apparatus according to claim 7, wherein the lower limit voltage value of the transfer bias is decreased.   In the job, when the interval between each recording material after the (N + 1) th sheet and the preceding recording material is less than a predetermined value in the job, the transfer unit with respect to each recording material after the (N + 1) th sheet is performed. When the transfer bias for transfer is changed with respect to the transfer bias for the transfer to the Nth recording material and the interval is equal to or greater than the predetermined value, each of the recording materials after the N + 1th recording material The image forming according to claim 1, wherein a transfer bias for the transfer with respect to the N-th recording material is not changed with respect to the transfer bias for the transfer with respect to the Nth recording material. apparatus.   In the job, when the value correlated with the electrical resistance of the recording material preceding each of the N + 1th and subsequent recording materials exceeds a predetermined value in the direction in which the electrical resistance is low, When the transfer bias for the transfer with respect to each recording material after the first is changed with respect to the transfer bias for the transfer with respect to the Nth recording material, and the correlated value does not exceed the predetermined value The transfer bias for the transfer with respect to each of the N + 1th and subsequent recording materials is not changed with respect to the transfer bias for the transfer with respect to the Nth recording material. The image forming apparatus according to any one of 1 to 9.   The image forming apparatus according to claim 1, wherein N is one.   12. The conductive member according to claim 1, wherein the conductive member is disposed on an upstream side of the fixing nipping portion and a downstream side of the transfer nipping portion in a recording material conveyance direction. Image forming apparatus.   The image forming apparatus according to claim 12, wherein the conductive member is a guide member that guides a recording material conveyed from the transfer holding unit to the fixing holding unit.   The image forming apparatus according to claim 1, wherein the fixing unit includes a heating source that generates heat when an AC voltage is applied thereto.   The image forming apparatus according to claim 1, wherein a direct current voltage is applied to the transfer unit.

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